1. Field of the Invention
This invention relates to liquid phase molybdenum catalyzed epoxidation reactions; and, more particularly, to the liquid phase epoxidation of olefins with molecular oxygen in the presence of an oxidized alkyl molybdate complex catalyst material.
2. Prior Art
Oxiranes or epoxides, while being valuable commercial products in and of themselves, are also commercially valuable as starting reactants for synthesizing, for example nontoxic antifreeze and urethane grade polyols. Over the years, many methods have been disclosed for synthesizing such compounds. The majority of these methods involve the oxidation of the corresponding olefin. For example, it is known that ethylene can be converted to the corresponding epoxide by a vapor phase partial oxidation with molecular oxygen over silver catalyst. The ease of olefin oxidation, however, varies greatly depending upon the size and structure of the olefinic starting reactant. Further, it has been disclosed that molecular oxygen can be utilized in the liquid phase epoxidation of olefins. Additionally, various catalysts have been found to expedite these epoxidation reactions.
Recently, it has been disclosed that molybdenum and/or tungsten containing compounds are effective in the liquid phase molecular oxygen epoxidation of olefins. Specifically, soluble molybdenum and tungsten catalytic substances of naphthenates, stearates, octoates and carbonyls are disclosed as being particularly preferred molybdenum and tungsten compounds for epoxidation of olefins. See for example Koller et al (French Pat. No. 1,459,880).
Additionally, it is known that metal compounds of titanium, zirconium, hafnium, vanadium, niobium, tantalum, molybdenum and tungsten are generally effective as catalysts in the liquid phase catalytic oxidation of olefins with molecular oxygen. Metal compounds such as oxides, acids including heteropoly acids or salts, or organic esters thereof, salts of organic acids, hydroxides, hydrated oxides, inorganic salts, organic complexes, carbonyls, and anhydrides are all disclosed as being suitable. See generally Allison (U.S. Pat. No. 3,259,638). In general, however, these compounds, and particularly the organic-based materials, are the simple metal compounds having one or two metal moieties per molecule. Additionally, the majority of these compounds are substantially insoluble in the reaction medium and generally show a low rate of reactant conversion to olefin when used as a catalyst. For example, in Table 1 of Allison, it is shown that all of the metal compounds were substantially inferior in conversion to vanadium metal itself.
Giovanni A. Bonetti et al (U.S. Pat. No. 3,480,563) discloses organic-soluble molybdenum compounds derived from a direct reaction between molybdic oxide and an alcohol. Molybdenum trioxide is taught as the only preferred starting material.
The above catalyst materials suffer from various disadvantages, including poor solubility in the reaction medium, and a low metal concentration. Thus, only small amounts of a catalyst metal are carried to the reaction medium in the catalytic compound. A large excess of the catalyst is then required to give acceptable reaction times. A large amount of catalyst material has been shown detrimental to the remaining steps of the process. Additionally, these compounds evidently undergo some decomposition at higher temperatures, causing a catalyst sludge or residue to form in the reactor. This sludge formation not only decreases catalyst activity, but presents a clean out problem which contributes substantially to the down time of the production facility. Further, many of the above catalyst compounds are known to form sludge and decomposition products upon standing at room temperature. Additionally, many of these catalyst compositions are not reproducible. Thus, different results are obtained when using a like catalyst from different batches or production lots.
Unexpectedly, it has now been found that certain molybdenum containing substances are extremely effective in catalyzing liquid phase epoxidation of an olefin with molecular oxygen to yield the corresponding oxirane. Surprisingly, these substances, which may be generally characterized as oxidized alkyl molybdate complexes, expedite a high conversion of the olefin to the corresponding oxirane, are unexpectedly stable at higher reaction temperatures and produce minimal amounts of sludge or residue in the reactor. Additionally, because of the ability of these substances to withstand relatively higher reaction temperatures, excellent yields of the desired product are obtained at relatively higher temperatures than heretofore known. Further, the method of the instant invention is highly economical owing to good conversion rates utilizing very short holding times.